Flowmeters are well known. One particular type of flowmeter is the fluidic oscillator flowmeter which is especially suited to the measurement of the flowrate of gases and liquids, particularly for domestic gas and water purposes. Such devices have been widely reported. Examples of such fluidic oscillator flowmeters can be found in both the academic and patent literature, for example those by Bauer in U.S. Pat. No. 4,244,230 or Herzl in U.S. Pat. No. 4,550,614. Such devices as shown schematically in FIG. 1 typically consist of a reducer section (1) which creates a jet of fluid (2) feeding into a diffuser section (3) which has first and second diffuser walls (4),(5) and first and second feedback channels (6),(7) respectively associated with the first and second walls which enable some of the jet flow to be fed back to the exit of the reducer section.
The mechanism for the flow measurement in a fluidic oscillator of this type has been described by several authors and is well understood. By means of the Coanda effect the jet naturally attaches to one wall or the other. Flow from the jet is then supplied to the feedback channel associated with the diffuser wall to which the flow is attached and the feedback flow feeds a separation bubble between the jet and the diffuser wall and forces the jet away from the wall towards the other diffuser wall. A splitter (8) or target may be used to speed the transition between the two walls. The jet then attaches to the other wall and the process repeats. The jet thus oscillates from one side of the fluidic oscillator to the other. The flow is measured by measuring the frequency of oscillation of the jet. This can be undertaken by the measurement of periodic variations in pressure or velocity at various points within the diffuser section, feedback channels or exit of the fluidic oscillator. For conducting fluids for example it is possible to employ inductive sensing techniques for the measurement of the velocity variations as identified by Sanderson and Heritage in European Patent 0381344. The various methods of measurement of these fluctuations are known to those versed in the art.
Typically such fluidic oscillators when used for domestic gas and water flow measurement have oscillation frequencies in the range 0.25 Hz to 100 Hz. Associated with the sensing technique there are electronic signal processing circuits which amplify and condition the signal to create a digital signal whose frequency corresponds to the frequency of oscillation of the fluidic oscillator. This digital signal is then fed into digital signal processing circuitry whose function is to convert the frequency into a flowrate or totalised flow. The methods of signal processing and of digital processing are known to those versed in the art.
The linearity of the frequency of oscillation to the flowrate of such devices and the low flowrate performance is dependent on the geometry of the fluidic oscillator and both these can be improved by using a conditioning element at the entrance of the fluidic oscillator. An example of such conditioning is given by Sanderson and Furmidge in European Patent 0868652.
Currently fluidic oscillator flowmeters only measure flow over the whole range accurately in one direction. In situations where there is the possibility of reverse flows occurring these would not be measured accurately and it is preferable that these flows should be prevented from flowing through the meter. In domestic water metering applications this is achieved by the installation of non-return valve downstream of the meter so that no flow can occur in the reverse direction.